55 堆 積 学 研 究,51号,55-66,2000 J.Sed.Soc.Japan,No.51,55-66,2000 Sedimentary environments in Lake Pukaki, New Zealand Augustine K. Chikita*, Ian Halstead** and Glenn Carter*** Sedimentary conditions in glacier-fed Lake Pukaki (20km long, 5km wide, and 98.0m deep at maximum), New Zealand were examined in the glacier-melt season of 1998. Spatial distributions of water temperature and suspended sediment concentration (SSC) obtained by a TTD (temperature-turbidity-depth) profiler indicate that sediment-laden underflow initiated near the river mouths is relatively weak because of their vertical low SSC on the gentle slope, and that the underflow is bifurcated into suspension interflow and other underflow at midlake ca. 1500m distant from the river mouths. The continu- ous measurement of water temperature at some depths revealed that at midlake, the suspension interflow and underflow continuously occur about one day at least. Key words: glacier-fed lake, sedimentation, suspension flow, sediment-laden underflow, river sediment discharge Tekapo, New Zealand by analyzing seismic refle- INTRODUCTION ction and sedimentary facies of surface sediment There exist some glacier-fed lakes on the and cores. southeast mountainside in the South Island, New In this study, characteristic behaviors of sus- Zealand. Of them, Lake Pukaki has the largest pension flow in Lake Pukaki are described by glacier cover (19.0%) in the drainage basin. Sed- obtaining spatial distributions of temperature imentation in the lake could thus be dominated and suspended sediment concentration of lake by glacier-melt sediment discharge of inflowing water and by analyzing grain size of surface Tasman River in summer (cf. Pickrill and Irwin, sediment. 1983). As representative hydrodynamic agents STUDY AREA AND METHODS of sedimentation in glacier-fed lakes, suspension overflow, suspension interflow and/or sediment- Lake Pukaki,South Island, New Zealand (44•‹4' laden underflow have been observed commonly S, 170•‹10'E ; surface area, 150km2 at mean lake (Gilbert,1975; Sturm and Matter, 1978; Weirich, level of 525m asl) receives large water supply in 1986; Chikita, 1992; Chikita et al., 1996; Chikita summer, due to ice-melt and snowmelt in et al., 1999). Pickrill and Irwin (1983) revealed Tasman Glacier, Hooker Glacier, Mueller Glacier, the sedimentary structure in glacier-fed Lake and other smaller drainage basins (Fig. 1). The largest inflowing river, Tasman River, then pro- Received: April 6, 2000, Accepted: April 28, 2000 vides the sediment discharge to the lake, and * Division of Earth and Planetary Sciences , Graduate consequently has developed the outwash plain School of Science, Hokkaido University, Sapporo 060- 0810, Japan: by delta progradation at the uplake end (Fig. 1). ** Environmental Data Division , National Institute of The total glacial cover occupies 19.0% of the Water and Atmospheric Research (NIWA) Ltd., P.O. drainage area (1027km2, excluding the lake sur- Box 29, Tekapo, New Zealand ***National Institute of Water and Atmospheric Re- face). Ice-contact lakes, dammed up by end mo- raines and/or outwash heads, exist on the termi- search (NIWA) Ltd., P.O. Box 8602, Christchurch, New Zealand nals of Tasman, Hooker, Mueller, and Murchison 56 Augustine K.Chikita,Ian Halstead and Glenn Carter 2000 Fig. 1 Location and drainage basin of Lake Pukaki. The terminals of Tasman, Hooker and Mueller Glaciers follow aerial photographs taken in December 1994 (Department of Survey and Land Information, 1996). Glaciers (Fig. 1; Warren and Kirkbride, 1998). flow of lake water thus possibly have no high These lakes seem to play a role as settling ponds suspended sediment concentration (probably, of suspended sediment produced from the gla- the order of 100mg/L at most), though the ciers (e.g., see Chikita et al., 1999). Tasman fluvial entrainment of deposits could occur on River and Hooker River initiated by the over- the outwash plain where Tasman River is J.Sed.Soc.Japan,No.51 Glacier-fed Lake Pukaki,New Zealand 57 braided. 3 is probably due to the boat's drift during the Sediment infilling in Lake Pukaki prevails in water sampling. half the lake basin proximal to the mouths of the Bulk density, ƒÏTC, of lake water at suspended braided river, as shown by the bathymetry in sediment concentration, C (mg/L), and tempera- Fig. 2. The development of the subaqueous ture, T (•Ž), was calculated by the following delta suggests (1) that the lake level greatly equation. varies throughout the year, (2) that the sediment ƒÏ TC-(1-C•~10-3/ƒÏs)ƒÏT+C•~10-3, (1) input to the lake prevails in the season of the low where ƒÏT is the pure water density in kg/m3 at lake level (possibly, early spring when snowmelt temperature, T (•Ž), and Ps is the sediment densi- begins), and (3) that suspension flows continu- ty (= 2779 kg/m3 for Pukaki). Finally, lake- ously occur far away to the subaqueous delta water density, ƒÏPTC (kg m-3) at water pressure, P, front (near a line of site D2 to site D1 in Fig. 2). was calculated by adopting the compressibility In order to examine sedimentary conditions in of 0.00045 per equivalent pressure (P=980000 Pa) the lake in the glacier-melt season, vertical pro- of 100m depth at an observed temperature range files of water temperature and suspended sedi- of 10 to 15•Ž. The water pressure, P, at a water ment concentration were obtained every 0.2m in depth, z, is given by the following equation. depth at 13 points in Fig. 2 on 10 and 11 January (2) 1998, using a TTD profiler (Alec Electronics Co., Ltd, Japan; model ATU-200; accuracies of •} where g is the gravitational acceleration, ƒ¢zi=zi 0.05•Ž, •}4ppm and •}0.22m for temperature, -zi -1 is the interval between two water depths, zi turbidity and water depth, respectively). The and zi-1, ƒÏTCi-(ƒÏTCi+ƒÏTCi-1)/2, ƒÏTCi is the water profiler was lowered from a boat positioned by a density at zi, and ƒÏTC0 is the water density at the portable GPS. We then spent 3 to 9 mins to get surface, Z0 (=0). vertical profiles at each point. The water Continuous measurements of water tempera- turbidity measured was converted into sus- ture were carried out at 15min intervals for a pended sediment concentration (SSC), using a period of 7 to 11 January 1998 at depths of 10.5m, relation between turbidity and SSC (Fig. 3). 20.5m, 30.5m, 37.5m, and 38.2m of site D by moor- The SSC was obtained by filtering lake-water ing temperature data loggers (ONSET Computer samples with millipore filters of 0.45ƒÊm opening. Inc., U.S.A.; accuracy of •}0.2•Ž). At sites A, C, The water sampling was performed with a Van D, F and G, bottom sediment was sampled by the Dorn water sampler on a boat immediately after Ekman-Birge grab sampler. The sediment was the vertical measurements of water turbidity. analyzed for grain size by the gravitational set- The relatively low correlation of r=0.6810 in Fig. tling method (grain size, d•…<44ƒÊm) and sieving (d Fig. 2 Location of observation sites on the bathymetric map of Lake Pukaki, made by NIWA, New Zea- land. The water depths (m) as shown by isoplethic lines correspond to those at a lake level of 524m asl. 58 Augustine K. Chikita, Ian Halstead and Glenn Carter 2000 Fig. 3 Relation between water turbidity (ppm) by the profiler and simultaneous suspended sediment concentration (mg/L) from water sampling. >44ƒÊm). tion (SSC) of Tasman River and Hooker River The water discharge of Tasman River to Lake upstream of their confluence was comparable to Pukaki was estimated by water budget of the each other at the roughly same time (Table 1). lake, because it is difficult to measure in the The SSC and water temperature seem to de- braided river (Figs. 1 and 2). SSC of inflow-river crease with decreasing natural river inflow (see water was obtained from manual water sam- Chikita et al., 1998). SSC and water discharge of pling at four points (black circles in Fig. 1). rivers except the glacial rivers is likely to be negligibly small as shown by those of Jacks RESULTS AND DISCUSSION Stream in Table 1 (see Fig. 1 for location). SSC Sediment Discharge of Tasman River of braided Tasman River flowing into Lake Figure 4 shows water level and natural river Pukaki is possibly comparable to that in up- inflow of Lake Pukaki at 1 h intervals for a stream Tasman and Hooker and at their confl- period of 0100h, 1 January to 0000 h,1 February uence, because the latter SSC could increase by 1998. The "natural river inflow" here indicates erosion of fluvial deposit, but decrease by the river water inflow from the natural drainage clear water inflow (SSC•`0mg/L) of Jollie River basin of the lake, mostly that from Tasman (see Fig. 1). Total sediment discharge of River (Fig. 1). This inflow was estimated by Na- braided Tasman River was thus estimated at 74.7 tional Institute of Water and Atmospheric Re- kg/s and 43.7kg/s, using the values of upstream search (NIWA), New Zealand, adopting the water Tasman and the confluence in Table 1, respec- budget of the lake. It is, however, under- tively. During our observation, sediment dis- estimated because the evaporation from the lake charge of braided Tasman River was relatively surface is neglected. As a result, the inflow of biased to the left coast of Lake Pukaki (see the Tasman River to Lake Pukaki for the observa- arrows in Fig. 2). This was also judged by flow tion period (7 to 11 January 1998) ranged from ca.